This invention relates to optical disks such, for example, as a CD, a CD-ROM, a CD-R, a CD-RW, a CD-I, a MD and a DVD and, more particularly, to optical disks having two areas of different recording densities. This invention relates also to optical disks having areas of different functions and to a method of manufacturing such optical disks and also to a method of recording information on such optical disks.
Optical disks such as a CD, a CD-ROM and a CD-R having a standard recording density have a track pitch of 1.6 .mu.m and a linear velocity of 1.2 m/s to 1.4 m/s. With the recent tendency to using a laser beam spot of a smaller diameter, optical disks having a higher recording density have been developed. For example, a DVD-ROM has a track pitch of 0.74 .mu.m and linear velocity of 3.94 m/s and has a recording capacity which is seven times as large as a CD.
In case a large amount of data such as a moving picture data is to be recorded in coping with the tendency to increasing the recording capacity of an optical disk, an optical disk having a high recording density may be used.
An optical disk drive device which can cope with media having a high recording density can drive also an optical disk having a lower recording density whereas an optical disk drive device which can reproduce only a CD having a standard recording density cannot drive an optical disk having a high recording density. For this reason, when an optical disk having a high recording density is used, conventional CD drive devices cannot reproduce such optical disk so that drive devices which can reproduce it are limited.
It is, therefore, a first object of the present invention to provide an optical disk which has a much larger recording capacity than a conventional optical disk of a standard recording density and yet is applicable also to a drive device which can reproduce only the conventional optical disk of the standard recording density.
Known also in the art of optical disks is one having areas of different functions. For example, Japanese Patent Application Laid-open 9-7223 discloses an optical disk having a ROM area used for read-only purpose and a RAM area which can be rewritten. This optical disk has advantages of both a read-only disk and a rewritable disk in that a large amount of copies can be produced from a software by using the ROM area while data can be written in the RAM area.
In this type of optical disk, information recorded in the ROM area cannot be erroneously overwritten but information recorded in the RAM area can be erroneously overwritten. In a floppy disk, overwriting can be inhibited but this can be made only for the entire disk and not for only a part of data which is important.
It is, therefore, a second object of the invention to provide an optical disk in which a recording mode can be selected depending upon the degree of importance of information.
It is a third object of the invention to provide a method for recording information on this optical disk.
It is a fourth object of the invention to provide a method for manufacturing this optical disk.